Heat grill to 400°F. Sprinkle ½ teaspoon blackened seasoning on one side of fish, then add a pinch or two of salt. Lay fish, seasoned side down, on sheet of foil just large enough to enclose fish. Add seasoning, salt, and two tablespoons butter to the top of fish. Fold top and sides of foil; make sure fish is completely enclosed. Grill for 7-10 minutes. Do not turn over. Fish is done when it's white and flaky. Note: Carefully open foil; it may contain extremely hot juices.